1. Field of the Invention
The present invention relates to an image formation apparatus and a control method thereof, and, more particularly, concerns an image formation apparatus that is provided with a rotary member and belts, such as a belt for transporting paper toward the rotary member or a transfer belt for allowing an image formed on the surface of the rotary member to be transferred on its own surface, and a control method of such an apparatus. Moreover, the present invention also relates to an image formation apparatus provided with a photosensitive drum and a belt, such as a transporting belt for transporting paper or an intermediate transfer belt, and an image formation method of such an apparatus, and a controlling device that is applicable to a mechanism that moves both a rotary member, which normally has eccentricities or deviations in its diameter, and a belt in an integral manner.
2. Discussion of the Background
In recent years, there is an ever-increasing demand for color image formation apparatuses capable of forming color images. One type of image formation apparatuses capable of forming color images at high speeds has an electrophotographic system of a tandem type. With respect to conventional image formation apparatuses of the tandem type, for example, the inventions disclosed in Japanese Patent Application Laid-Open Nos. 63-81373 and 10-246995 have been known.
Each of the inventions disclosed in the above-mentioned publications, Japanese Patent Application Laid-Open Nos. 63-81373 and 10-246995, has four photosensitive drums each of which has a scanning unit for applying a laser light beam to each of the four photosensitive drums so as to write a latent image thereon.
The four photosensitive drums are placed in parallel with each other in the transporting direction of the paper, which is transported by the transporting belt. Each of these is scanned (main-scanning process) in the direction of the rotary axis by a laser light beam directed from the scanning unit, while being rotated, so that a latent image is written thereon. Here, one line of the latent image written by one main-scanning process is hereinafter referred to as a scanning line.
On the surfaces of the four photosensitive drums bearing the latent images written thereon are supplied toners having respective colors of Y (yellow), M (magenta), C (cyan) and K (black) so as to adhere to the respective latent images. Thus, a toner image having one of the colors is formed on each of the surfaces of the four photosensitive drums. A sheet of paper is successively pressed onto the four photosensitive drums on which toner images have been formed. Consequently, the toner images of the respective colors are successively transferred the sheet of paper to form a color image.
In this case, if there is an offset between the scanning lines constituting the toner images of the respective colors in the color image thus formed, a so-called color offset will occur in the color image, resulting in degradation in the image quality. In order to prevent the color-offset, Japanese Patent Application Laid-Open No. 10-246955 has proposed an arrangement in which the photosensitive drums are designed to rotate freely, while an annular transport belt is driven to rotate by a motor so that the transport belt is made to contact the photosensitive drums by press-contact rollers installed below the transport belt. The four photosensitive drums are driven to rotate, following the transport belt. At this time, the four photosensitive drums are subjected to the same rotary force so that they are allowed to rotate at the same angular velocity, thereby making it possible to form a color image that is free from positional offsets between the scanning lines.
However, in image formation apparatuses such as printers and copying machines, there will be ever-increasing demands for high resolution (1200 dpi or more).
In contrast, the technique disclosed in Japanese Patent Application Laid-Open No. 10-246995 fails to meet these demands. In other words, in the technique disclosed in Japanese Patent Application Laid-Open No. 10-246995, the image distortion in the sub-scanning direction due to the eccentricity of the rotary axis of the photosensitive drum is corrected by detecting a outer surface dislocation and using the dislocation information continuously. However, when a high resolution is required, the actual amount of correction tends to deviate from the dislocation information in their correlation as the resolution becomes higher.
Moreover, in general, the photosensitive drum tends to have a slight eccentricity due to the limitation in its assembling precision. FIG. 43 is a drawing that shows a state in which a transfer belt is made to contact a photosensitive drum 1801 having such an eccentricity by a contact roller. The photosensitive drum 1801 shown in this Figure has a cross-section that is orthogonal to the rotary axis perpendicular to the paper surface, that passes through point O.
The photosensitive drum 1801 having the eccentricity rotates centered on the center axis passing through point O. In contrast, a transport belt 1802, which has an annular shape, is allowed to move in the direction of arrow A. A contact roller 1803 is made to contact the transport belt 1802 from below while being supported by a spring 1804 so that the transport belt 1802 is made to press-contact the photosensitive drum 1801. A sheet of paper, not shown, is made to press-contact the photosensitive drum 1801 by the press-contact roller 1803 through the transport belt 1802. Thus, a toner image formed on the surface of the photosensitive drum 1801 is transferred onto the sheet of paper.
The distance from point O to the outer surface of the photosensitive drum 1801 having the eccentricity varies depending on the angle of rotation when observed at a fixed point. For this reason, when the center of gravity of the photosensitive drum 1801 is located at G1, the paper and the photosensitive drum 1801 are in contact with each other at press-contact position P1, while when the center of gravity of the photosensitive drum 1801 is located at G2, they are in contact with each other at press-contact position P2.
The press-contact roller 1803 is allowed to move up and down to a certain degree since this is supported by a spring 1804. Since the press-contact roller 1803 also has an eccentricity, the press-contact position varies in a complex manner, thereby giving greater adverse effects to the angular velocity of the photosensitive drum 1801.
When the angular velocity of the photosensitive drum 1801 varies, the distance between scanning lines of a latent image to be written on the photosensitive drum 1801 becomes irregular, resulting in a distortion in the image to be formed. Moreover, in the case of a color image formation apparatus of the tandem type having a plurality of photosensitive drums in which multi-color toner images are superposed so as to form a color image, if the angular velocities of the photosensitive drums deviate, offsets occur in the transferring positions of the toner images of the respective colors, resulting in degradation in the image quality of an image to be formed.
Here, another arrangement has been proposed in which: the transferring position of a toner image is estimated through calculations, and based upon the results thereof, the image forming conditions are adjusted so as to make the transferring positions coincident with each other. However, when the angular velocity of the photosensitive drum 1801 varies, it becomes difficult to accurately estimate the transferring positions of the toner images, resulting in failure to adjust the transferring positions of the toner images through adjustments of the image formation apparatus. Therefore, in the image formation apparatus in which the angular velocity of the photosensitive drum varies, it is impossible to improve the image quality by eliminating the offsets in the transferring positions of the toner images through the adjustments of the image forming conditions.
Moreover, the color image formation apparatus of the tandem type is provided with a writing unit for each of the photosensitive drums. Here, the writing timings and properties of the optical systems in the respective writing units placed in the respective photosensitive drums are not necessarily coincident with each other. For this reason, the writing timing deviates for each of the photosensitive drums, and even when there is no eccentricity in each of the photosensitive drums, there might be deviations in the transferring positions for the toner images of the respective colors.
Furthermore, the radius of each of the photosensitive drums tends to have a slight deviation due to the limitation in the processing precision. In this case also, deviations tend to occur in the transferring positions in the toner images of the respective colors, regardless of the eccentricity of each of the photosensitive drums.
In the future, along with the ever-increasing demands for high-quality printing with high resolution (1200 dpi or more), very high-precision photosensitive drums have to be produced in order to solve the above-mentioned problems. Taking into consideration the development of the technology in the future, it will be possible to improve the processing precision of the photosensitive drum to a certain extent; however, there will be a limitation in the improvement of the processing precision.
One of the objectives of the present invention is to provide an image formation apparatus which, even when high resolution is required for forming an image, sufficiently prevents a distortion and a color offset in the sub-scanning direction of an image, and makes deviations in the sub-scanning pitch due to the eccentricity of the photosensitive drum less conspicuous so that image data in the sub-scanning direction is outputted in the same timing as the case having neither eccentricity in the photosensitive drum nor deviations in the drum, and makes it possible to cut production costs and also to form correction data for correcting distortions and color offsets in the image in the sub-scanning direction with high precision.
Moreover, another objective of the present invention is to provide an image formation apparatus and a control method for the image formation apparatus, in which, independent of the states of respective image formation apparatuses such as eccentricity due to deviations at the time of assembling photosensitive drums, deviations in the writing timing of writing units and deviations in the radii of the photosensitive drums, toner images of the respective colors are transferred on a sheet of paper on the transfer belt without positional offsets, thereby making it possible to form high-quality images.
Furthermore, still another objective of the present invention is to provide an image formation apparatus and a control method for the image formation apparatus, in which independent of the states of respective image formation apparatuses such as eccentricity due to deviations at the time of assembling photosensitive drums and deviations in the radii of the photosensitive drums, toner images of the respective colors are transferred on a sheet of paper on the transfer belt without positional offsets, thereby making it possible to form high-quality images.
An image formation apparatus in accordance with the present invention is provided with: a photosensitive drum, an optical writing device which carries out an optical writing process in the main-scanning direction on the outer surface of the photosensitive drum at least line by line, a developing device which develops an electrostatic latent image optically written on the photosensitive drum by using toner, a transport member that is allowed to move in synchronism with the photosensitive drum while being pressed onto the photosensitive drum to transport a sheet of paper so that the toner image on the photosensitive drum is transferred on the sheet, or an intermediate transfer belt which transfers the toner image located on the photosensitive drum, that is shifted in synchronism with the photosensitive drum while being pressed onto the photosensitive drum, a rotation angle detection unit which detects the rotation angle of the photosensitive drum directly or indirectly, an eccentricity detection unit which detects the eccentric position from the rotary axis of the photosensitive drum located in the center of the circle cross-section of the photosensitive drum, and a correction unit which, based upon the results of detection by the rotation angle detection unit and the eccentricity detection unit, finds the amount of eccentricity of the photosensitive drum, the eccentric rotation angle and the radius of the photosensitive drum, and based upon the resulting values, corrects a distortion and a color offset in the toner image that has been transferred on the sheet or the intermediate transfer belt. With this arrangement, based upon the amount of eccentricity of the photosensitive drum, the eccentric rotation angle and the radius of the photosensitive drum, the distortion and color offset in the toner image that has been transferred are corrected so that, even when high resolution is required for forming an image, it is possible to sufficiently prevent the distortion and color offset in the sub-scanning direction of the image.
Moreover, another image formation apparatus in accordance with the present invention is provided with: at least one rotary member such as a photosensitive drum that is pressed onto the belt directly or indirectly and allowed to integrally rotate with the shift of the belt; a belt driving unit which moves the belt, or a rotary member driving unit which rotates the rotary member such as the photosensitive drum; and at least one of a rotary member load correction unit for correcting variations in the load imposed on the rotary member such as the photosensitive drum and a belt load correction unit for correcting variations in the load imposed on the belt. With this arrangement, it is possible to regulate the variations in the load imposed on the rotary member such as the photosensitive drum or the belt, and consequently to reduce the amount of the load variations transmitted to the belt such as the transport belt or the intermediate transfer belt; therefore, it becomes possible to prevent a slip from occurring between the belt or the sheet of paper and the rotary member such as the photosensitive drum.
Moreover, a control method for an image formation apparatus in accordance with the present invention, which is a control method for an image formation apparatus that is provided with a plurality of photosensitive drums for forming images, has at least one of an eccentricity detection step of detecting the eccentricity of each photosensitive drum, a measuring step of measuring the radius of each photosensitive drum and a distance detection step of detecting the distance between the photosensitive drums, and a detection function, placed at a certain portion thereof, for detecting the rotation angle of each photosensitive drum or for detecting the shift of the belt; thus, the positioning of the rotation angle is carried out independently for each photosensitive drum. With this arrangement, although variations in density occur in each latent image formed on the photosensitive drum due to the eccentricity of each photosensitive drum, a toner image, formed on a sheet of paper or the intermediate transfer belt by superposing the latent images, has a state of variations in density that are virtually made coincident with each other; thus, it becomes possible to obtain a high-quality image.
Moreover, still another image formation apparatus in accordance with the present invention, which is an image formation apparatus that uses a plurality of photosensitive drums for forming images, has at least one of an eccentricity detection unit which detects the eccentricity of each photosensitive drum, a measuring unit which measures the radius of each photosensitive drum and a distance detection unit which detects the distance between the photosensitive drums, and a detection unit, placed at a certain portion thereof, for detecting the rotation angle of each photosensitive drum or for detecting the shift of the belt; thus, the positioning of the rotation angle is carried out independently for each photosensitive drum. With this arrangement, although variations in density occur in each latent image formed on the photosensitive drum due to the eccentricity of each photosensitive drum, a toner image, formed on a sheet of paper or the intermediate transfer belt by superposing the latent images, has a state of variations in density that are virtually made coincident with each other, thus, it becomes possible to obtain a high-quality image.
Another control method for an image formation apparatus of the present invention, which is a control method for an image formation apparatus having at least one rotary member such as a photosensitive drum that is pressed onto the belt directly or indirectly and allowed to integrally rotate with the shift of the belt; and a belt driving unit which moves the belt, or a rotary member driving unit which rotates the rotary member such as the photosensitive drum, is provided with at least one of a rotary member load correction step of correcting variations in the load imposed on the rotary member such as the photosensitive drum and a belt load correction step of correcting variations in the load imposed on the belt. With this arrangement, it is possible to regulate the variations in the load imposed on the rotary member such as the photosensitive drum or the belt, and consequently to reduce the amount of the load variations transmitted to the belt such as the transport belt or the intermediate transfer belt; therefore, it becomes possible to prevent a slip from occurring between the belt or the sheet of paper and the rotary member such as the photosensitive drum.
Moreover, still another image formation apparatus in accordance with the present invention, which is an image formation apparatus having at least one rotary member that is pressed onto a belt directly or indirectly and allowed to rotate following the shift of the belt and a driving roller for driving the belt, is provided with a driving-roller drive unit which drives the driving roller, a rotary-member driving unit which drives the rotary member, a load variation detection unit which detects load variations in the belt and a control unit which controls the operation of the driving-roller drive unit or the rotary-member driving unit in accordance with variations in the load of the belt. The present invention is supposed to be used for an image formation apparatus provided with at least a rotary member that is pressed onto an annular belt for transporting, for example, a sheet of copy paper, directly or indirectly, with the paper interpolated in between, and allowed to move following the shift of the belt, and a driving roller for driving the belt. The driving-roller drive unit drives the belt. Moreover, the rotary-member driving unit drives the rotary member. The load variation detection unit detects the load variation imposed on the belt. The control unit controls the operations of the driving-roller drive unit and the rotary-member driving unit in accordance with variations in the load of the belt. With this arrangement, it is possible to detect a load transmitted from the rotary member to the belt, and based upon the results of the detection, the driving operation of the rotary member is controlled so as to cope with the entire load imposed on the driving roller, thereby making it possible to eliminate a slip on the belt. Therefore, the image formation apparatus is readily applied to a high-quality printing operation.
Furthermore, still another image formation apparatus in accordance with the present invention, which is provided with at least a rotary member that rotates while being pressed onto the belt directly or indirectly, a velocity detection unit which detects at least one of the shifting velocity of the belt and the velocity related to the rotary member, and a control unit, and in the system having the velocity detection unit and the control unit, a velocity setting unit which sets the shifting velocity of the belt and the rotation velocity of the rotary member to velocities that allow the belt and the rotary member to move integrally is further installed. With this arrangement, even when there is a variation in the shape of each rotary member, it is possible to prevent a slip from occurring between the rotary member and the belt, between rotary member and the sheet of paper, or between the sheet of paper and the belt.
Moreover, still another image formation method in accordance with the present invention, which is an image formation method applied to an image formation apparatus having at least one rotary member that is pressed onto a belt directly or indirectly and allowed to rotate following the shift of the belt and a driving roller for driving the belt, is provided with a driving-roller driving step for driving the driving roller, a rotary-member driving step for driving the rotary member, a load variation detection step which detects load variations in the belt and a control step for controlling the operation of the driving roller or the rotary member in accordance with variations in the load of the belt. The present invention relates to an image formation method for controlling an image formation apparatus provided with at least a rotary member that is pressed onto an annular belt for transporting, for example, a sheet of copy paper, directly or indirectly, with the paper interpolated in between, and allowed to move following the shift of the belt, and a driving roller for driving the belt. The driving-roller driving step drives the belt Moreover, the rotary-member driving step drives the rotary member. The load variation detection step detects the load variation imposed on the belt. The control step controls the operations of the driving roller and the rotary member in accordance with variations in the load of the belt. With this arrangement, it is possible to detect a load transmitted from the rotary member to the belt, and based upon the results of the detection, the driving operation of the rotary member is controlled so as to cope with the entire load imposed on the driving roller, thereby making it possible to eliminate a slip on the belt. Therefore, the image formation apparatus is readily applied to a high-quality printing operation.
Furthermore, still another image formation method in accordance with the present invention, which is an image formation method for controlling at least a rotary member that rotates while being pressed onto the belt directly or indirectly, is provided with a velocity detection step which detects at least one of the shifting velocity of the belt and the velocity related to the rotary member, and a control step, and in the system having the velocity detection step and the control step, a velocity setting step for setting the shifting velocity of the belt and the rotation velocity of the rotary member to velocities that allow the belt and the rotary member to move integrally is further provided. With this arrangement, even when there is a variation in the shape of each rotary member, it is possible to prevent a slip from occurring between the rotary member and the belt, between rotary member and the sheet of paper, or between the sheet of paper and the belt.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.